Process for the recovery of aromatic hydrocarbons from mixtures containing the same

ABSTRACT

A process is disclosed whereby aromatic hydrocarbons such as benzene, toluene and xylene are separated with high selectivity from mixtures in which they are contained through extraction and/or extractive distillation with the aid of a solvent mixture including morpholine and another solvent such as an oxygen containing derivative of morpholine and water, e.g. morpholinewater, morpholine-N-formyl morpholine and morpholine-N-formyl morpholine-water, in three main steps: (1) concentration of nonaromatic hydrocarbons to produce a stream free from aromatics; (2) purification of aromatics dissolved in rich solvent; and (3) recovery of the aromatics from the rich solvent.

I United States Patent 1 [111 3,816,302 Paret June 11 1974 PROCESS FOR THE RECOVERY OF 2,850,461 9/1958 Bloch et a1 208/323 O C HYDROCARBONS FROM 3,262,875 7/1966 Girotti et a1. 208/326 3,325,399 6/1967 Cinelli et a1. 208/326 MIXTURES CONTAINING THE SAME 3,434,936 3/1969 Luther et al. 208/313 [75] Inventor: Giancarlo Paret, Milan Italy 3,679,579 7/1972 Preusser et al. 208/323 [73] Asslgnee: afig ig fg; San Donate Primary ExaminerDelbert Ev Gantz Assistant ExaminerC. E. Spresser [22] Filed: Dec. 29, 1971 Attorney, Agent, or FirmRa1ph M. Watson, Esq. [21] Appl. No: 213,567

[57] ABSTRACT [30] F i A li ti P i it D t A process is disclosed whereby aromatic hydrocarbons Dec 29 1970 Italy 3361 5/70 such as benzene, toluene and xylene are separated with high selectivity from mixtures in which they are [52] U 5 Cl 208/323 208/313 208/326 contained through extraction and/or extractive distil- 6 /67 4 lation with the aid of a solvent mixture including mor- [511 Int Cl Clog 21/06 pholine and another solvent such as an oxygen con- [58] Field 326 taining derivative of morpholine and water, e.g. morpholine-water, morpholine-N-formyl morpholine and morpholine-N-formyl morpholine-water, in three main [56] References Cited steps: (1) concentration of non-aromatic hydrocarbons to produce a stream free from aromatics; (2) pu- UNITED STATES PATENTS rification of aromatics dissolved in rich solvent; and

as 61 208/326 (3) recovery of the aromatics from the rich solvent. ers erger 2,357,667 9/1944 Kuhn 208/326 7 Claims, 2 Drawing Figures PATENTEDJUNH 1914 v 3L816l302 sum 1 or 2 Fig.1

PROCESS FOR THE RECOVERY OF AROMATIC HYDROCARBONS FROM MIXTURES CONTAINING THE SAME The present invention relates to a process for the recovery of aromatic hydrocarbons from mixtures containing the same. More particularly the present invention relates to a process for the recovery of benzene, toluene, xylene and aromatics C from mixtures containing the same, obtaining aromatics at the purity levels required by the petrochemical uses.

The above mentioned process makes use as means of extraction and/or of extractive distillation, of a solvent constituted by a mixture containing morpholine, the remaining part of the mixture being constituted by one or more than one solvents, comprising among them also water. More particularly the present invention makes use, as means of extraction and/or of extractive distillation, of mixtures of morpholine in the presence of one or more solvents among which mention is made of acetonitrile, furfural, aniline, dimethyl formammide, dimethylacetamide, N-methyl pyrrolidone, ,B-methoxypropionitrile, sulpholane, glycols as diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol, singly or in mixtures, dimethylsulfoxide and others among which according to the process of the present invention the oxygenated compounds of morpholine are of particular importance. As oxygenated compounds of morpholine one can advantageously use all the compounds having an oxygen containing group bound to the morpholine ring.

In particular mention is made of N-formylmorpholine, 2-formyl-morpholine, 3-formylmorpholine, morpholine-acetone and the like. As noted above, one of the constituents of the solvent mixture can be also water, in addition to morpholine.

Particularly interesting in the process according to the present invention have been the mixtures morpholine-water, morpholine N-formyl morpholine (which in the following description we will call formylmorpholine) and morpholine-formyl morpholinewater, even though the mixtures of morpholine with one or more than one of the previously noted solvents are always valid.

With particular reference to the morpholine-formyl morpholine, morpholine-water and morpholine-formyl morpholine-water mixtures, they present very good solvent and selectivity properties; said properties can be possibly improved by a possible addition of one or more than one of the other mentioned solvents in variable amounts.

It is important to underline the fact that, morpholine being a remarkably stable and inert compound, it can be mixed with water without corrosion and decomposition risks. Furthermore it is known that morpholine has very good properties as corrosion inhibitor in corrosion conditions.

The very high selectivity of the solvent mixtures easily allows one to recover products of high purity. Another important feature according to the present invention is the fact that with the use of the described solvent mixtures the column and in particular the reboiler, can work at low temperatures. Moreover the high solvent power allows one to maintain low solvent/hydrocarbon ratios, even if a high recovery of aromatics is effected.

The whole involves a lesser consumption of the total steam and also avoids the risks of solvent degradation. A further advantage is that of requiring the use only of low pressure steam, reducing in such a way the working costs.

As regards the compositions of the solvent mixtures used in the process according to the present invention, morpholine is always present in concentrations higher than 0.5 percent by weight, up to 99 percent by weight. Water can be used in concentrations ranging from 1 to 35 percent, preferably between 1 and 20 percent, the complement to 100 of the whole mixture being constituted by one or more than one of the other mentioned solvents.

In the particular case of oxygenated compounds of morpholine and more particularly of formyl morpholine, the concentration may range from 1 to 98.5 percent by weight and preferably between 1 and 50 percent by weight.

The solvent mixtures previously described can be preferably but unrestrictively used for recoverying aromatics, in particular benzene, toluene, xylene and aromatic hydrocarbons having a higher number of C atoms, from mixtures containing the same either by extraction or by distillation or by a combination of the two operations.

According to various possible processes,-the operation of recovery of aromatics from mixtures containing the same is carried out in three main steps:

1. Concentration of the non aromatic hydrocarbons so as to produce a stream free from aromatics.

2. Purification of the aromatics dissolved into the rich solvent so eliminating the other components.

3. Recovery of the aromatic from the rich solvent.

As an example but unrestrictively, reference is made to the apparatus shown in FIG. 1 by means of which it is possible to effect the aromatics recovery according to the above mentioned three steps:

The first step and a part of the second step are effected by liquid-liquid extraction and said extraction operations (production of a raffinate without aromatics and partial purification of the extract) are effected in only one apparatus 1. The feed 2 enters the extraction column at about half height; the solvent 3 is fed to the top of the extraction column. The raffinate 4, poor in aromatics and containing some dissolved solvent is discharged from the top of the extraction column and is fed to a scrubbing column 17 for the recovery of the solvent by means of water 5 thus obtaining a purified raffinate 19.

The extract 6 constituted by the solvent containing the aromatics is discharged from the bottom of extraction column 1; the reflux 7 constituted by a mixture of saturated and aromatic hydrocarbons coming from the extractive distillation column 8 and by a part of the water coming from the column 17 for the raffinate scrubbing, enters the bottom (of column 1). The reflux has the purpose of changing the composition of the saturated hydrocarbons present in the solvent rich stream running downwards along the column, removing the heavier hydrocarbons which can hardly be removed in the subsequent extractive distillation. Water has an analogous effect since it lowers in a more emphasized way the solubility of the more heavy saturated hydrocarbons in the solvent running downwards along the column.

The extract obtained from the extraction column bottom, constituted by a mixture of aqueous solvent, aromatics and relatively light saturated hydrocarbons is fed to the top of the extractive distillation column 8. In said column, owing to the presence of the solvent which increases the volatility of the non aromatic hydrocarbons, there is the separation by distillation of the light saturated hydrocarbons from the aromatics. This column works without external reflux and at atmospheric pressure; the feed is preheated by making use of the sensible heat of the solvent leaving the bottom of the aromatics recovery column 10. The overhead product 11, constituted by hydrocarbons with a little water, is condensed at two different thermal levels; a small fraction 12 (about 10 percent) is condensed at a higher temperature and fed to the extraction column together with the feed; the remaining portion 13 is fed to the bottom of the extraction column as reflux 7; in this way we obtain an adjunctive removal of the heaviest non aromatic hydrocarbons and the attainment of the required purities is easier. The small amount of water which condenses together with the hydrocarbon usually is not separated and is recycled together with the same to the extraction column l.

The bottom product 14 of the extractive distillation column is constituted by solvent and pure aromatics and is fed to the column l for the recovery of the aromatics. In the aromatics recovery column the feed enters the column some plates below the top plate; the highest plates have the purpose of removing the solvent contained in the vapour phase so that the overhead product 16 would not contain solvent. Since the overhead product is constituted by water and aromatics 18, the water is of such a purity that it can be used for washing the raffinate in 17 without supplementary manipulations.

The temperature of the column bottom is related to the composition of the solvent mixtures and to the pressure; with the formyl morpholine-morpholine-water mixtures usually utilized, the temperature of -the bottom of the stripping column is maintained at such values that it is possible to make use of low pressure steam, working with the condenser at atmospheric pressure.

The solvent 9 leaving the column bottom is free from aromatics. It can be cooled, firstly by heat exchange with the water leaving the column top, thus contemporaneously producing steam which is introduced into the bottom of the same column, then by heat exchange with the solvent 6 leaving the extraction column and by heat exchange with cooling water down to the working temperature of extraction column l.

The solvent mixtures of the present invention can be advantageously utilized in other processes and in differ.- ent points of the same mixtures, different both in composition and in components, can be used.

An example will be given for illustrative but unrestrictive purposes.

EXAMPLE With reference to the enclosed FIG. 2, an artificial hydrocarbon mixture 13 was fed to a liquid-liquid extraction column l at a rate of l kg/h; the feed had the following composition (by weight):

by weight The rich solvent leaving the bottom of extraction column l was fed, after heating, to the 30th plate of the extractive distillation 2 at the temperature of C. The column worked without external reflux, the pressure at the column top being the atmospheric pressure.

The distillate was condensed in two steps. The condensate of the 1st step 3 10 percent by weight of the total) equal to 0.036 kg/h, constituted by:

benzene 0.015 toluene 0.005 nheptane 0,016

was recycled together with the feed. The remaining portion (4) equal to 0.324 Kg/h of the following composition:

benzene 0,185 toluene 0.005 n-heptane 0.134

was fed, as reflux, to the bottom of the extraction column (1).

The rich solvent 5 leaving the bottom of the extractive distillation column 2 was then fed to the 25th plate of the aromatics separation column 6, having 40 plates and working with atmospheric pressure at the top.

The overhead vapours were condensed totally and separated in two phases. The hydrocarbon phase 7 (aromatic extract) leaving the separator had the following composition:

benzene 0.498 kg/h toluene 0.0% kg/h The content of non aromatic hydrocarbons in the toluene obtained by means of conventional fractionation from the aromatic extract, was lower than 200 p.p .m.

The melting point of benzene was 5.45C

The aqueous phase 8 obtained by decantation of the condensate of the vapours leaving the top of the stripping column (0.1 kg/h) was divided in two portions:

a stream 9 (0.03 kg/h) was fed as reflux to the 40th plate of the stripping column.

a stream (0.07 kg/h) was fed to the column 11 for the washing of the raffinate wherein, by countercurrent liquid-liquid extraction, it extracted all morpholine and formyl morpholine dissolved in the hydrocarbon phase.

The aqueous solution 12 coming out from 11 was then fed, as aqueous reflux, to the bottom of the aromatics extraction column 1. By cooling with water the temperature of the lean solvent leaving the bottom of the stripping column 6 was reduced to room temperature before the same was fed to the 60th plate of the aromatics extraction column.

What we claim is:

1. Process for the recovery of aromatic hydrocarbons from a mixture containing aromatic and non-aromatic hydrocarbons comprising:

a liquid-liquid extraction step wherein said hydrocarbon mixture is contacted with a solvent mixture of from 0.5 to 99 percent by weight of morpholine, from 1 to 98.5 percent by weight of a formyl morpholine and the balance to 100 percent by weight of water, to produce a raffinate substantially free from aromatic hydrocarbons and an extract rich in aromatic hydrocarbons;

an extractive distillation step wherein light saturated hydrocarbons in said extract are separated from the aromatic hydrocarbons; and

a stripping step wherein said aromatic hydrocarbons are separated from solvent and recovered at a high degree of purity.

2. Process according to claim 1 wherein said formyl morpholine is N-formyl-morpholine.

3. Process according to claim 2 wherein N-formylmorpholine constitutes from 1 to 50 percent of the solvent mixture. 7

4. Process according to claim 1 wherein said formyl morpholine is 2-formyl-morpholine.

5. Process according to claim 1 wherein said formyl morpholine is 3-formyl-morpholine.

6. Process according to claim 1 wherein water constitutes from 1 to 20 percent of the solvent mixture.

7. Process according to claim 1 wherein said aromatic hydrocarbons are comprised of benzene, toluene, xylene and aromatic hydrocarbons having nine or more carbon atoms. 

2. Process according to claim 1 wherein said formyl morpholine is N-formyl-morpholine.
 3. Process according to claim 2 wherein N-formyl-morpholine constitutes from 1 to 50 percent of the solvent mixture.
 4. Process according to claim 1 wherein said formyl morpholine is 2-formyl-morpholine.
 5. Process according to claim 1 wherein said formyl morpholine is 3-formyl-morpholine.
 6. Process according to claim 1 wherein water constitutes from 1 to 20 percent of the solvent mixture.
 7. Process according to claim 1 wherein said aromatic hydrocarbons are comprised of benzene, toluene, xylene and aromatic hydrocarbons having nine or more carbon atoms. 